The present invention relates to an electric power steering apparatus using a brushless motor to provide assistant steering force to a steering mechanism in an automotive vehicle.
Conventionally, there has been used an electric power steering apparatus which drives an electric motor in accordance with steering torque provided by a driver to a handle (steering wheel) so as to provide assistant steering force to a steering mechanism in an automotive vehicle. As the electric motor of the electric power steering apparatus, a brush motor has been used widely. Recently, a brushless motor has been used in view of enhancing reliability and durability or reducing inertia.
In addition, in an electric power steering apparatus mounted on a full-sized vehicle or a medium-sized vehicle, a driving current of the electric motor generating the assistant steering force may increase excessively and thus causing a problem of generating heat resulting from the motor driving current. In order to protect electronic components included in the electric power steering apparatus, it is required to suppress the amount of heat resulting from the motor driving current so as to be not greater than a predetermined value.
Accordingly, in the field of the electric power steering apparatus, there has been known about a method of estimating the amount of heat from an actual current value flowing in the electric motor so as to restrict the motor driving current on the basis of the estimated amount of heat. For example, in JP2003-284375A, there is disclosed a method of estimating a coil temperature on the basis of the square sum of the values of two-phase current acquired by converting the level of three-phase current flowing in the electric motor into two-phase current. In JP2002-19635A, there is disclosed a method of integrating the squared values of the motor driving current. In JP2002-362392A, there is disclosed a method of calculating a maximum current value on the basis of the coefficients of the motor driving current and detected heat.
However, in the electric power steering apparatus having a brushless motor, there is performed a control in which the level of the three-phase current is changed to form a sinusoidal wave form with delaying the phases of the three phase current are delayed by 2π/3 one another in accordance with the rotation position (angle) of a rotor of the brushless motor. Accordingly, an excessively large amount of current may concentrate in a specific phase depending on the rotation position of the rotor and thereby increasing the amount of heat in that phase.
For example, as shown in FIG. 6, in the case of changing the three-phase current flowing in the brushless motor in accordance with the rotation position θ of the rotor, the rotor stopped at a position corresponding to θ=5π/6 (or the rotor slowly rotated with very slow speed in an almost halt state). In this case, an absolute value of a w-phase current Iw approximately corresponds to the maximum current value Imax. Accordingly, the amount of heat in the w-phase becomes greater than that for the case where the w-phase current is alternating like the sinusoidal wave.
In such an electric power steering apparatus having the brushless motor, as the rotation speed of the rotor decreases, irregularity or variation of the amount of heat occurs among the three phases. Accordingly, even when the motor driving current is restricted on the basis of the amount of heat estimated from the current flowing in the brushless motor without discriminating respective phases, an excessively large amount of current may concentrate in a specific phase and thereby increasing the amount of heat in that phase. Accordingly, it is difficult to properly protect electronic components included in that phase.